Various methods have been proposed for enabling a detection of the approach of a conductive trigger. For example, in the eddy current method, eddy current losses produced by a trigger in an alternating magnetic field are evaluated. The alternating magnetic field is usually generated by an oscillator with a LC oscillating circuit, which reacts to eddy current losses by the reduced quality of the LC circuit. On reaching a switching criterion, the resultant change in the oscillation amplitude is detected by an evaluation circuit to control a load switch.
Inductive proximity switches based on the eddy current method have been proposed in a number of variations such as, for example in DE-AS 1 286 099.
One disadvantage of proximity switches employing the eddy current method resides in the fact that triggers with different conductivities produce eddy current losses of different magnitudes and hence result in different response distances of the proximity switch. The response distance is reduced, for example, by a reduction factor of 1/2 to 1/3 for nonferrous metals in contrast to magnetic steels.
Moreover, in the eddy current method, in addition to the "desired" losses, there are also "parasitic" losses, for example, losses through the winding resistance or in the ferrite material and in the sealing compound which are largely dependent on ambient temperature.
In order to attempt to compensate for the temperature dependency, proximity switches have been proposed in, for example, EP-OS 0 070 796 and DE-OS 3 814 131, wherein special measures are employed to reduce temperature dependence; however, these proposed switches involve considerable technical circuit cost.
It has also been proposed to avoid a change in material-dependent response distance in a proximity switch by using the change in permeability of an LC circuit as a function of the frequency to respond to the approach of the trigger. The change in coil inductance is evaluated as well as the eddy current losses. With correct component values, especially when using highly stable components, it is possible to obtain equal and nearly temperature-independent response distances for ferrous and nonferrous metals. Temperature-independent response distances which exceed the coil diameter cannot be obtained over a wide temperature range, however.
Finally, inductive proximity switches have been proposed in DE-OS 3 840 532, in which two spatially separated sensing coils measure the power loss in the alternating field itself and evaluate it. Although, a proximity switch of this type eliminates the influence of parasitic losses in the response distance, the direct measurement of the sensing signal, using unloaded field-measuring coils, has proven to be very sensitive to interference, in contrast to electromagnetic interference. Furthermore, the circuit expense for the required link between an additional required reference signal and the sensor signal is very high.